We have investigated point defects in GaN grown by HVPE by using steady-state and time-resolved photoluminescence (PL). Among the most common PL bands in this material are the red luminescence band with a maximum at 1.8 eV and a zero-phonon line (ZPL) at 2.36 eV (attributed to an unknown acceptor having an energy level 1.130 eV above the valence band), the blue luminescence band with a maximum at 2.9 eV (attributed to ZnGa), and the ultraviolet luminescence band with the main peak at 3.27 eV (related to an unknown shallow acceptor). In GaN with the highest quality, the dominant defect-related PL band at high excitation intensity is the green luminescence band with a maximum at about 2.4 eV. We attribute this band to transitions of electrons from the conduction band to the 0/+ level of the isolated CN defect. The yellow luminescence (YL) band, related to transitions via the −/0 level of the same defect, has a maximum at 2.1 eV. Another yellow luminescence band, which has similar shape but peaks at about 2.2 eV, is observed in less pure GaN samples and is attributed to the CNON complex. In semi-insulating GaN, the GL2 band with a maximum at 2.35 eV (attributed to VN) and the BL2 band with a maximum at 3.0 eV and the ZPL at 3.33 eV (attributed to a defect complex involving hydrogen) are observed. We also conclude that the gallium vacancy-related defects act as centers of nonradiative recombination.

Results for long-wavelength emitters are presented for semi-polar InGaN/AlGaN/GaN heterostructures grown on
GaN(1122)/m-sapphire templates by metalorganic chemical vapor deposition. The semi-polar GaN layers were 10 to 25
μm thick and grown by HVPE on sapphire substrates. X-ray diffraction measurements indicated high crystallographic
quality that approaches that of GaN(0001) layers on sapphire. A comparison based on optical pumping experiments,
low- and high-density excitation photoluminescence experiments, and atomic force microscopy is drawn between
InGaN/GaN quantum well laser heterostructures grown by metalorganic vapor phase epitaxy either on either polar
GaN(0001)/c-sapphire or on semi-polar GaN(1122)/m-sapphire. C-plane InGaN/GaN/sapphire structures exhibited low
threshold pump power densities < 500 kW/cm2 for emission wavelengths up to 450 nm. For laser structures beyond 450
nm the threshold pump power density rapidly increased resulting in a maximum lasing wavelength of 460 nm. Semipolar
InGaN/GaN(1122)/m-sapphire structures showed a factor of 2-4 higher threshold pump power densities at
wavelengths below 440 nm which is partly due to lower crystalline perfection of the semi-polar GaN/sapphire templates.
However, at longer wavelengths > 460 nm the threshold power density for lasing of semi-polar heterostructures is less
than that for c-plane heterostructures which enabled rapid progress to demonstration of lasing at 500 nm wavelength on
semi-polar heterostructures. The absence of V-type defects in semi-polar, long-wavelength InGaN/GaN structures which
are usually present in long-wavelength c-plane InGaN/GaN structures is attributed to this phenomenon.

Optically-based chemical and biological sensors require optoelectronic devices with specific emission and detection
wavelength ranges. Semiconductor optoelectronic devices applicable to this sensing are of particular interest due to their
low power consumption, compact size, long lifetime, and low cost. We report the electrical and optical properties of
deep UV p-i-n photodiodes (PDs) based on short period superlattices (SPSLs) of AlN/AlGaN. All device and test
structures are grown by gas source molecular beam epitaxy with ammonia on sapphire and AlGaN/sapphire substrates.
AlGaN/sapphire substrates were grown by stress controlled hydride vapor phase epitaxy (HVPE). The cutoff
wavelength of PDs based on these SPSLs can be varied from 250 to 280 nm by changing the SPSL barrier/well
thickness ratio. For mesa diodes with 150 μm diameter we obtain extremely low dark leakage current of ~ 3 pA/cm2,
and high zero-bias resistance of ~ 6 x 1014 Ω. A cutoff wavelength of 247 nm is obtained for these devices with four
orders of magnitude rejection by 315 nm. We obtain a maximum responsivity of 60 mA/W.

We report the electrical and optical properties of deep ultraviolet light emitting diodes (LEDs) based on digital alloy structures (DAS) of AlN/Al0.08Ga0.92N grown by gas source molecular beam epitaxy with ammonia on sapphire substrates and AlGaN/sapphire templates. AlGaN/sapphire templates were grown by recently developed stress controlled hydride vapor phase epitaxy (HVPE). For DAS with effective bandgap of 5.1 eV we obtain room temperature electron concentrations up to 1x1019 cm-3 and hole concentrations of 1x1018 cm-3. Based on these results we prepared double heterostructure (DHS) LEDs operating in the range of 250 to 290 nm. The emission wavelengths were controlled through the effective bandgap of the active region. The possible ways for increase of LED's efficiency are discussed. We observed significant improvement in the room temperature luminescence efficiency (by factor of 100) of AlGaN quantum wells when a transition growth mode is induced by reduced flux of ammonia. We found that active layer grown on HVPE AlGaN/sapphire substrates have higher luminescence efficiency (by factor of 3) than DAS grown on sapphire.

In this paper we report on the fabrication and characterization of GaN, AlGaN, and AlN layers grown by hydride vapor phase epitaxy (HVPE). The layers were grown on 2-inch and 4-inch sapphire and 2-inch silicon carbide substrates. Thickness of the GaN layers was varied from 2 to 80 microns. Surface roughness, Rms, for the smoothest GaN layers was less than 0.5 nm, as measured by AFM using 10 μm x 10 μm scans. Background Nd-Na concentration for undoped GaN layers was less than 1x1016 cm-3. For n-type GaN layers doped with Si, concentration Nd-Na was controlled from 1016 to 1019 cm-3. P-type GaN layers were fabricated using Mg doping with concentration Na-Nd ranging from 4x1016 to 3x1018 cm-3, for various samples. Zn doping also resulted in p-type GaN formation with concnetration ND-NA in the 1017 cm-3 range. UV transmission, photoluminescence, and crystal structure of AlGaN layers with AlN concentration up to 85 mole.% were studied. Dependence of optical band gap on AlGaN alloy composition was measured for the whole composition range. Thick (up to 75 microns) crack-free AlN layers were grown on SiC substrates. Etch pit density for such thick AlN layers was in the 107 cm-2 range.

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